1. Field of the Invention
The present invention relates to a semiconductor device having capacitor elements. In particular, the present invention relates to a high-dielectric-constant film and a ferroelectric film which is made by using an organometal material gas, and is used for a capacitor or a gate of a semiconductor integrated circuit.
2. Description of the Related Art
In recent years, nonvolatile ferroelectric memories, which utilizes a ferroelectric capacitor, and dynamic random access memories (DRAMs), which utilizes a high-dielectric capacitor, have been actively investigated and developed. These ferroelectric memories and DRAMs comprise selective transistors and data-storing capacitors, and in a memory cell a capacitor connected to one diffusion layer of the selective transistor. A ferroelectric capacitor uses a ferroelectric film made of Pb(Zr,Ti)O3 (hereinafter, referred to as xe2x80x9cPZTxe2x80x9d) etc., as a capacitor insulating film, and thereby storing non-volatile information by polarizing a ferroelectric.
On the other hand, a high-dielectric capacitor uses a high-dielectric thin film made of (Ba,Sr)TiO3 (hereinafter, referred to as xe2x80x9cBSTxe2x80x9d) etc., as a capacitor insulating film. Therefore, the capacitance of the capacitor can be increased to miniaturize a device. When such a ceramic material is used for a semiconductor device, it is very important to electrically isolate such a ceramic material of each memory cell as a minute capacitor. A ceramic material film is deposited on a crystallize-assisting conductive film that is to be a lower electrode.
As a method for depositing ceramic thin film, a conventional sol-gel process, sputtering, and chemical vapor deposition (CVD) have been reported.
The sol-gel process is a method in which organometallic materials dissolved in an organic solvent are applied to a wafer having a lower electrode formed thereon by a spin-coating, and crystallized by annealing in the presence of oxygen. This process causes crystallization in a solid phase, so that a very high temperature is required for crystallization. In the case where a metal oxide dielectric film is PZT, a crystallization temperature that allows sufficient ferroelectric characteristics to be exhibited is 600xc2x0 C. In the case where the metal oxide dielectric film is BST, a crystallization temperature that allows sufficient high-dielectric characteristics to be exhibited is 650xc2x0 C. A crystal formed by the foregoing method has many defects, and uneven crystal orientation. Furthermore, according to this process, it is difficult to handle a large-diameter wafer, and step-coverage is poor. Thus, the sol-gel process is not suitable for fabrication of highly integrated devices.
Next, the sputtering method is a method in which a crystal is formed on a wafer having lower electrode film, by reactive sputtering using Ar+O2 plasma and a ceramic sintered-body target, and then subjected to crystallization by anealing in oxygen. According to sputtering, film uniformity could be obtained by enlarging the diameter of a target, and a sufficient deposition rate is obtained by increasing a plasma injection power. However, sputtering also has a drawback that a high temperature is required for crystallization. In the case where a metal oxide dielectric film is PZT, a crystallization temperature that allows sufficient ferroelectric characteristics to be exhibited is 600xc2x0 C. In the case where the metal oxide dielectric film is BST, a crystallization temperature that allows sufficient high-dielectric characteristics to be exhibited is 650xc2x0 C. Furthermore, according to sputtering, a composition is almost determined by that of a target; therefore, a target is required to be exchanged in order to vary the composition of a film, which is inconvenient in terms of production steps.
Next, according to CVD, source gases are introduced in a gas phase into a deposition chamber wherein a heated substrate is set, then a film is formed on the substrate. CVD has excellent deposition characteristics in uniformity of a large-diameter wafer and a step-coverage, so that it is considered to be promising method as a mass-production technique when applied to ULSI. Organometals are often employed for CVD source because there are few appropriate hydrides and chlorides for constituent metal element of ceramics including Ba, Sr, Bi, Pb, Ti, Zr, Ta, and La, and so on. However, organometals have rather low vapor pressure, and in most cases, present as a solid or a liquid at room temperature, so that a transportation method utilizing a carrier gas is used.
However, in the case of using the above-mentioned method, it is difficult to quantify the flow rate of an organometal gas in a carrier gas and to control the flow rate thereof exactly. More specifically, the carrier gas contains an organometal source gas with a saturation vapor pressure (determined by the temperature of a source vessel) or higher, and this flow rate depends upon not only the flow rate of a carrier gas but also the surface area of a solid material, the temperature of a thermostat, and the like. Furthermore, according to the description on film formation of PTO (lead titanate: PbTiO3) using the above-mentioned film formation method, in Jpn, J. Appl. Phys. Vol. 32 (1993) p.4175), a film formation temperature of PTO is also very high (570xc2x0 C.), and crystal orientation is not uniform.
Hitherto, for formation of a ferroelectric memory and a DRAM, the above-mentioned film formation method has been used. However, heating at a high temperature of about 600xc2x0 C. or higher in an oxygen atmosphere is necessary, and it is also difficult to control crystal orientation.
According to a structural aspect of a semiconductor device, in order to make a ferroelectric capacitor and a high-dielectric capacitor well functioning, it is required to electrically connect either one of capacitor electrodes to diffusion layers of a selective transistor. Conventionaly, in a DRAM, polysilicon layer connected to one diffusion layer of a selective transistor is used as lower electrode of a capacitor, and a SiO2 film, a Si3N4 film, or the like is formed as an insulating film of the capacitor on the surface of the polysilicon layer to construct the capacitor. However, since a ceramic thin film is made of an oxide, if the ceramic thin film is formed directly on polysilicon, the polysilicon is oxidized. Thus, a structure in which a ceramic film directry formed on a polysilicon electrode cannot be obtained. Therefore, as a one alternate structure, a cell structure in which an upper electrode of a capacitor and a diffusion layer are connected to each other by local wiring of metal made of Al or the like is described in 1995 Symposium on VLSI Technology Digest of Technical Papers, p. 123. Furthermore, according to International Electron Devices Meeting Technical Digest, 1994, p. 843, a technique for forming a PZT capacitor by using TiN barrier metal on polysilicon is described. Regarding a DRAM, for example, according to International Electron Devices Meeting Technical Digest, 1994, p. 831, a technique is described for forming a STO (strontium titanate: SrTiO3) thin film on a RuO2/TiN lower electrode formed on a polysilicon plug, thereby forming a capacitor.
On the other hand, Japanese Patent Application Laid-open No. 11-317500 discloses a memory cell structure in which a capacitor is connected to a diffusion layer by alternately stacked plugs and metal pads formed simultaneously with a multi-layered metal line, unlike a conventional memory cell structure in which a capacitor is connected to a diffusion layer via local wiring and a polysilicon plug.
Incidentally, perovskite crystal has two a-axes (which may be referred to as an a-axis, b-axis) and a c-axis, as shown in FIG. 14. A polarization direction in which ferroelectricity is exhibited is a c-axis direction, and polarization does not occur in an a-axis direction. In case of a thin film, Ferroelectricity is exhibited by a component of the c-axis in a direction vertical to the film surface. Thus, the ratio of polycrystalline grains in the a-axis orientation and the c-axis orientation in the film has a large effect on ferroelectric characteristics of a ferroelectric film. As a proportion of grains oriented in the c-axis direction is increased, a magnitude of spontaneous polarization becomes larger.
Japanese Patent Application Laid-open No. 2000-58526 discloses a method for introducing an organic metal material gas and an oxide gas from separate inlet ports to a vacuum chamber, and forming a metal oxide dielectric film under a total pressure of 1xc3x9710xe2x88x922 Torr or less in the vacuum chamber. According to this method, even if a metal oxide dielectric thin film is formed at a low substrate temperature of 450xc2x0 or lower, a film with good crystallinity having less degradation of characteristics is obtained. Therefore, after an aluminum line or a transistor is formed on an underlying layer, a ferroelectric capacitor can be formed above a substrate.
However, even with a PZT film described in JP 2000-58526 A, the ratio of grains in the a-axis orientation and the c-axis orientation of perovskite crystal may not be controlled.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a metal oxide dielectric film with excellent ferroelectricity, in which orientation ratio between an a-axis and a c-axis is controlled.
The present invention relates to a metal oxide dielectric film containing perovskite crystal represented by ABO3, wherein a composition ratio between an A-element and a B-element contained in the film satisfies the following Equation (1-1), and an amount of an oxide of the A-element contained in the film satisfies the following Equation (1-2):
1 less than [A]/[B]xe2x89xa61.1xe2x80x83xe2x80x83(1-1)
where [A]/[B] represents a composition ratio (atomic ratio) of the A-element with respect to the B-element in the metal oxide dielectric film;
(IAO/IABO3) less than 10xe2x88x922xe2x80x83xe2x80x83(1-2)
where IAO and IABO3 respectively represent (111) peak intensity of an oxide of the A-element and (100) peak intensity of the metal oxide dielectric in an X-ray diffraction spectrum of the film.
In the case where the metal oxide dielectric film is a PZT film, the composition of Pb, Zr, and Ti contained in the film satisfies the following Equation (2-1), and the amount of PbO contained in the film satisfies the following Equation (2-2):
1 less than [Pb]/([Zr]+[Ti])xe2x89xa61.1xe2x80x83xe2x80x83(2-1)
where [Pb]/([Zr]+[Ti]) represents a composition ratio (atomic ratio) of Pb with respect to the total of Zr and Ti in the PZT film;
(IPbO/IPZT) less than 10xe2x88x922xe2x80x83xe2x80x83(2-2)
where IPbO and IPZT respectively represent (111) peak intensity of PbO and (100) peak intensity of PZT in an X-ray diffraction spectrum of the PZT film.
It is preferable that the metal oxide dielectric film is formed by thermal CVD by heating a substrate provided in a vacuum chamber, while introducing an organic metal material gas and an oxide gas from separate inlet ports into the vacuum chamber.
At this time, the total pressure in the vacuum chamber is preferably 1xc3x9710xe2x88x922 Torr or less. Furthermore, it is preferable that a substrate temperature during the film formation is 450xc2x0 C. or lower.
It is preferable that the oxide gas includes particularly nitrogen dioxide gas.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.